Hermetic motor compressor crankcase venting system



e. T. JENSEN 3567393 HERMETIC MO I'OR COMPRESSOR CRANKGASE VENTINGSYSTEM Aprii 21, 1970 Filed Dec; 10, 1968 INVENTOR.

GUNTHER T. JEN SEN HIS ATTORNEY United States Patent 3,507,193 HERMETICMOTOR COMPRESSOR CRANKCASE VENTING SYSTEM Gunther T. Jensen, Tyler,Tex., assignor to General Electric Company, a corporation of New YorkFiled Dec. 10, 1968, Ser. No. 782,570 Int. Cl. F04b 39/02 US. Cl. 92-153Claims ABSTRACT OF THE DISCLOSURE A hermetic compressor comprising ahermetic shell containing a body of oil in the bottom thereof and areciprocating compressor including a substantially closed crankcasepartially immersed in the oil has a restricted drain passageinterconnecting the lower portion of the crankcase with the body of oiland a restricted vent interconnecting an upper portion of the crankcasewith the interior of the shell above the level of the oil. The flowrestrictions of the drain passage and vent are selected to preventexcessive circulation of hot gases from the crankcase to the compressorshell as well as to substantially prevent two way flow of oil betweenthe crankcase and the oil sump during operation of the compressor.

BACKGROUND OF THE INVENTION Hermetic refrigerant compressors of the typewith which the present invention is concerned comprise a compressor forcompressing the refrigerant and a motor for driving the compressorsealed within a gas tight casing or shell. The compressor com rises acompressor block including a crankcase having axially aligned bearingsin the upper and lower walls thereof and at least one cylinder openinginto the crankcase. A vertically extending drive shaft journalled inthese bearings drives a piston slidably fitted within the cylinder froma drive motor positioned above the crankcase. At least the lower endportion or wall of the crankcase is immersed in a body of oil containedin a sump in the lower portion of the shell so that a suitable oilpumping means such as that described in Dubberley Patent 3,098,604 canbe used to convey oil from this sump to the upper bearing and to theconnecting rod bearing interconnecting the piston to the drive shaft. Assome of this oil seeps from the bearings into the crankcase, means inthe form of one or more drain passages are normally provided forreturning oil to the sump.

In one form of compressor of this type, the crankcase is open orsubstantially open to the interior of the shell above the oil level withthe result that the interior of the crankcase operates at substantiallythe same pressure as the shell. The drain passage or passages in thebottom wall of the crankcase serve only to return excess lubricant tothe sump and the oil level within the crankcase is the same as that inthe sump under all pressure conditions. In a second form of compressorof this type, the crankcase is closed so there is no communicationbetween the crankcase and the portion of the shell above the level ofthe oil. As a result, the level of the oil within the crankcase isgenerally dependent upon the relative pressures in the crankcase andshell. During operation of the compressor, compressed gas from thecylinder bypassing the piston increases the pressure within thecrankcase sufficiently to force oil or liquid refrigerant from thecrankcase into the sump. To prevent build up of crankcase pressure, thedrain passage or passages have to be large enough to vent all of thebypass gases to the shell and as a result are large enough to permit asubstantial alternating or pulsating flow of lubricant between thecrankcase and the sump under the effect of the fluctu- 3,507,193Patented Apr. 21, 1970 ating crankcase pressures caused by thereciprocating movement of the piston.

A general object of the present invention is to provide a hermeticcompressor comprising a combination of vent and drain means assuring aone way flow of bypass gas from the crankcase to the compressor shell,substantially eliminating any pulsating or two way flow of oil betweenthe crankcase and the shell and substantially preventing oil from beingdraWn into the crankcase as a result of a sudden increase in shellpressure. Further objects of the invention are the provision of ahermetic compressor unit including a substantially closed crankcasehaving vent and drain means for reducing the possibility of liquidsludging in the cylinder of the compressor during start up, forproducing controlled foaming of the oil for noise attenuation, and forincreasing compressor capacity and reducing power consumption.

To these ends, the illustrated compressor of the present inventioncomprises a hermetic casing or shell having an oil or lubricant sump inthe bottom thereof and containing a vertical shaft motor-compressor unitwith at least the lower crankcase portion of the compressor immersed inthe oil. The compressor component includes at least one cylinder openinginto the crankcase and the crankcase is closed except for a restricteddrain passage in the bottom wall thereof below the level of oil and acooperating restricting vent interconnecting the upper portion of thecrankcase with the shell above the level of oil. The combined andrelative flow restrictions of the vent and drain passage are such as toassure a continuous flow of refrigerant vapor and oil from the crankcaseto prevent the build-up of a substantial positive crankcase pressurewhile at the same time limiting pulsating flow of oil and refrigerantbetween the shell and crankcase.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, the singlefigure is a view, partly diagrammatic and partly in section, of arefrigeration circuit including a hermetic compressor of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis illustrated a hermetic compressor unit comprising a shell or casing 1in which is resiliently supported a motor compressor unit. This unitcomprises a compressor block 2 defining a substantially closed crankcase3 and a cylinder 4 opening into the crankcase. The compressor block alsoincludes upper and lower axially aligned bearings 6 and 7 in which ismounted a vertically extending shaft 8 having an eccentric bearingportion 9 between the bearings 6 and 7. A connecting rod 10 connects apiston 11 to the bearing 9. Thus piston 11 reciprocates or slides backand forth in the cylinder 4 in response to the reciprocating forcesprovided by the eccentric bearing 9 upon rotation of the shaft 8.

Means for driving the compressor comprises an electric motor 14positioned in the upper portion of the shell 1 above the compressorblock 2 and having a rotor 15 attached to the shaft 8.

The bottom of the shell 1 defines a sump for containing a body oflubricating oil 17 used to lubricate the various bearings. This body oflubricant is preferably of a suflicient depth that the lower end of thecrankcase including the bearing 7 is substantially immersed in the oiland is lubricated by such immersion. For lubrication of the upper mainbearing 6 and the eccentn'c bearing 9, there is provided a centrifugalpumping arrangement (not shown) including oil passages extendingupwardly through shaft 8 to these bearings as described in the aforemen-3 tioned Dubberley Patent 3,098,604. Oil seeping from the bearings flowsinto the crankcase 3.

The compressor is designed to form part of a hermetic refrigerationsystem including, as diagrammatically illustrated, a condenser 21, anexpansion device which may be either an expansion valve or, as shown, acapillary tube 22, and an evaporator 23 connected in closed series flowrelationship. During operation of the compressor low pressure or suctiongas is withdrawn from the evaporator 23 through an inlet 24 in the upperportion of the shell 1. This relatively cool suction gas passesdownwardly through the motor 14 and through a plurality of holes 25 intoan annular suction muflier 26 formed in the upper portion of thecompressor block 2. The suction gas flows from the muffler 26 throughone or more horizontal passages 27 into an annular cavity 28 surroundingthe foreward end of the cylinder 4 and from this cavity through aplurality of suction ports 29 and a suction valve into the interior orchamber 30 of the cylinder 4.

Refrigerant compressed by the reciprocating piston 11 flows through adischarge mufiler 31 into a discharge line 32 which includes a pluralityof loops 34 immersed in the body of oil 17 and is thereafter dischargedfrom the compressor unit through an outlet 35 to the condenser 21.

During operation of the compressor, the momentary pressure within thecrankcase is determined primarily by two diflferent conditions. Thefirst is that the fit between the piston, piston ring, and the walls ofthe cylinder permits compressed gas from the compression chamber 30 tobypass the piston 11 during each compression movement thereof therebycontinuously introducing high pressure gas into the crankcase. Thesecond condition is a fluctuating or pulsating change in the crankcasepressure resulting from the reciprocating movement of the piston 11within the cylinder 4. In other words, as the piston recip rocateswithin the cylinder, the pressure in the crankcase alternately increasesand decreases proportional to the change in the effective volume thereofas determined by the displacement of the piston.

In the operation of previously known compressors of the closed crankcasetype, the drain passage or passages for returning to the sump any oilcollecting in the crankcase were also large enough to vent all of thebypass gases from the crankcase. As a result, such a drain passage, whenopening below the level of the oil in the sump, also permitted analternating or two Way flow of oil between the sump and the crankcasedue to the fluctuating pres sure conditions within the crankcase. Thisconstant pumping of the oil caused a significant increase in the load onthe compressor due to the work required to pump the oil into and out ofthe crankcase through the drain passage, and also the work required tocirculate the oil around the inside of the crankcase due to the rotationof the crankshaft. On the other hand, in the operation of compressors ofthe open crankcase type, that is compressors in which the crankcase hada non-restricted connection with the interior of the shell above the oilsump, the oil in the crankcase remained at the same level as in the sumpsince the fluctuating or changing crankcase pressures were dissipated tothe larger shell volume. However, in such compressors, all of therelatively hot bypass gases also became mixed with the relatively coolsuction gas within the shell thereby increasing the temperature anddecreasing the density of the gas flowing to the cylinder chamber with aresultant decrease in the effective pumping capacity of the compressor.

In accordance with the present invention, the compressor crankcase isprovided with a combination of vent and drain means designed to providemany of the advantages of both the open and closed crankcaseco1npressors while minimizing the disadvantages thereof. To this end,there is provided a restricted vent passage 37 in the upper portion ofthe crankcase 3 interconnecting the interior of the crankcase with theinterior of the shell 1 above the level of the oil in the sump and arestricted drain passage 38 in a lower wall portion of the crankcaseopening into the shell below the normal level of the body of oil in theshell. The vent 37 is preferably located as high as possible in a sidewall of the crankcase above the cylinder and opens to the interior ofthe shell 1 outside the sleeve 39 supporting the motor stator above thecrankcase. The hole should also preferably be in the side wall so thatoil in the crankcase that is being rotated around the inside of thecrankcase due to rotation of the crankshaft will also be expelled outthe vent passage due to centrifugal force. The drain hole 38 is locatedas low as possible so that the maximum amount of oil can be displacedfrom the crankcase. The lower end 40 opens below the normal oil level inthe sump so that the exit noise of the oil and gas from the drainpassage will be mufiled by the sump oil and also cause agitation of thesump oil to produce foaming.

Broadly described, the flow restrictions of the vent and drain passages,or conversely their flow capacities, are such that under normalcompressor operating conditions, the vent 37 has a flow capacity whichis insufiicient to vent all of the bypass gas from the crankcase to theshell while the drain passage 38 has a flow capacity sufficient to ventthe remaining bypass gas the shell but insufficient to permit asubstantial two way flow of lubricant between the sump and the crankcaseas a result of the normal fluctuating pressure conditions with thecrankcase.

The advantages of this combination of vent and drain means and thedesired restrictions, or flow capacities thereof, will become apparentfrom a consideration of the operation of a compressor incorporating suchmeans.

During shut-down, the pressures within the refrigeration systemincluding the condenser 21, the evaporator 23 and the hermeticcompressor unit tend to equalize at a pressure intermediate theoperating or normal suction and condenser pressures. This equalizationof pressures also take place in all of the volume within the hermeticshell 1. Thus both the shell and the crankcase are at the sameintermediate pressure and the level of oil within the crankcase is thesame as the sump level as illustrated in the drawing. Usually, theliquid contained in the sump and the crankcase is a mixture of oil andliquid refrigerant with the oil containing some dissolved refrigerantWhile the shell and crankcase above the oil level contains refrigerantgas.

At start-up of the compressor, reciprocation of the piston within thecylinder causes a fluctuation of the crankcase pressure, the magnitudeof which depends upon the displacement of the piston as compared withthe overall volume of the crankcase. During a forward movement of thepiston there is an eflective increase in the crankcase volume and hencea decrease in the crankcase pressure so that some liquid refrigerant inthe crankcase or refrigerant dissolved in the oil within the crankcasevaporizes. During return movement, the piston is acting against thisincreased amount of gas with a proportional increase in crankcasepressure. Some of the vapor Within the crankcase is vented through thevent 37 and some of the liquid in the lower portion of the crankcase isdischarged through the drain passage 38. This process repeats until allof the liquid within the bottom of the crankcase has been dischargedtherefrom.

Thereafter during normal operation of the compressor, flow of bypass gasfrom the chamber 30 to the crankcase 3 will be vented to the shellthrough both the vent passage 37 and the drain passage 38. The gasflowing through passage 38 foams the oil below the compressor and aroundcoils 34 for decreasing transmission of compressor and discharge tubingnoise to the shell. Also oil seeping from the bearings within thecrankcase will continuously drain through the drain passage 38.

During shut-down of the compressor, liquid refrigerant and oil willaccumulate within the compressor shell 1, the crankcase 3, the suctionpassage 29 and the cylinder chamber 30. In accordance with one aspect ofthe present invention, the flow restrictions of the vent 37 and thedrain passage 38 are such that during start-up, the liquid in thecrankcase is forced out slowly, for example during the first five or tenseconds of operation of the compressor. During this period, and due tothe aforementioned pressure conditions within the crankcase, there is anincreased load on and hence a reduction in the speed of the piston. Therate of acceleration of the compressor is thus reduced so that theliquid in the cylinder can be expelled therefrom while the compressor isoperating at a relatively slow speed thereby reducing the possibility ofvalve failure by this liquid slugging in the cylinder.

The restricted drain passage 38 has the additional advantage ofpreventing a significant two way flow of oil between the sump and thecrankcase by the fluctuating pressure conditions within the crankcase.Once a positive mean pressure difference between the crankcase and theshell is established by the flow of bypass gas into the crankcase thereis no significant reverse flow of lubricant into the crankcase throughthe passage 38 during a decrease in the fluctuating pressure within thecrankcase.

The combination upper and lower vent and drain passages also prevents aharmful quantity of oil from being drawn into the crankcase under asudden rise in the shell or suction pressure. This is accomplished notonly by the flow restriction provided by the drain 38 but also by thepresence of the upper vent 37. Under a sudden rise in shell pressure,the vent 37 permits a reverse fiow of refrigerant vapor into thecrankcase thereby more quickly equalizing the crankcase and shellpressures and again establishing a pressure relationship between thecrankcase and the shell which will cause only a one-way flow of oil orgasout of the drain passage. Vent 37 also serves to equalize thecrankcase and shell pressures and the oil levels during shut-downperiods. Without vent 37, the increasing shell pressure duringequalization of suction and discharge pressures, will force oil into thecrankcase 3 through drain hole 38 until the pressure inside crankcase 3is equal to the pressure in the shell 1 so that the liquid level withinthe crankcase is then higher than in the shell.

Furthermore due to restriction of the vent 37, only a portion of therelatively warm bypass gas flows directly to the upper portion of theshell 1 so that only a fraction of the total bypass gas affects thetemperature of the suction gas within the shell and hence the compressorperformance. The remaining bypass gas is vented below the surface of theoil through the drain passage 38 and has the additional advantage ofwarming this body of oil thereby decreasing the solubility ofrefrigerant therein.

It will be obvious of course that the optimum flow restrictions or flowcapacities of the vent passage 37 and the drain passage 38 will dependupon a number of factors including the amount of piston blow by and therelationship between the total crankcase volume and the pistondisplacement volume. The combined flow restriction should be suflicientto maintain a slight but positive, for example from 0.1 to 5 pounds persquare inch, pressure differential between the mean crankcase pressureand the shell pressure under normal operating conditions. Preferablythis pressure differential is less than 2 pounds per square inch. Inother words, the restrictions should provide sufiicient flow of bothvapor and oil from the crankcase to prevent build-up of an excessivemean pressure difference while limiting any pulsating or alternatingflow of oil and refrigerant. Such alternating flow of oil or gas. aspreviously described, consumes power and preheats the suction gas.

From the above description, it will be obvious that the restriction ofthe vent passage 37 should be such that during the first few seconds ofoperation suflicient pressure difference between the crankcase and theshell will be established to force all of the liquid from the crankcasethrough the drain passage 38 while at the same time the drain passage 38should also have sufficient restriction to prevent substantialalternating flow of oil therethrough during normal compressor operation.To accomplish these objectives, a vent-to-drain passage flow restrictionratio of about 1:2 is presently considered to be particularly effectivefor most compressor displacements. With such a ratio, about /3 of thebypass gas is vented through the vent passage while the remaining /a isvented through the drain passage. In a compressor operating at 3600r.p.m. and having a crankcase volume of approximately 20 cu. in.(excluding the piston displacement volume) and a piston displacementvolume of approximately 3 cu. in., these results were obtained by usingvent and drain passages both having a length of about one inch with thediameter of the vent being 0.086 inch and the diameter of the drain0.111 inch. The invention is, of course, not limited to these specificdimensions. However, the length of the drain passage, and hence itslength-todiameter ratio, should be such that during decrease incrankcase pressure in normal compressor operation, the gas-oil interfaceremains within the drain passage so that oil does not flow from the sumpinto the crankcase.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A hermetic compressor comprising a hermetic shell containing a bodyof lubricant in the bottom thereof and a reciprocating compressormounted within said shell and having a portion submerged in said body oflubricant;

said compressor comprising a substantially closed crankcase, a cylinderopening into said crankcase and a reciprocating piston in said cylinderproviding a compression chamber, the fit between said piston andcylinder permitting some compressed gas in said chamber to bypass saidpiston and flow into said crankcase, reciprocating movement of saidpiston in said cylinder also effecting alternating increase and decreasein crankcase pressure;

a restricted drain passage interconnecting the lower portion of saidcrankcase with said body of lubricant below the surface thereof and arestricted vent interconnecting an upper portion of said crankcase withthe interior of said shell above the surface of said body of lubricant;

said vent having a flow capacity sufficient to vent only a portion ofthe bypass gas to said shell.

2. The compressor of claim 1 in which said drain passage has a flowcapacity sufficient to conduct the remaining bypass gas to said shell.

3. The compressor of claim 2 in which the flow restriction of said drainpassage is suflicient to prevent substantial flow of lubricanttherethrough into the crankcase due to pressure changes in saidcrankcase caused by said reciprocating movement of said piston.

4. The compressor of claim 2 in which said vent and drain passageprovide fiow restrictions such that liquid refrigerant and lubricant insaid crankcase are slowly discharged through said drain passage duringinitial startup of said compressor to thereby initially decrease theacceleration of said compressor.

5. The compressor of claim 3 in which said vent has a flow capacitysufiicient to substantially relieve a pressure difference between saidshell and said crankcase upon a sudden increase in the gas pressure insaid shell.

6. The compressor of claim 5 in which the relative flow restrictions ofsaid vent and drain passage are approximately 1:2.

7. The compressor of claim 2 in which said vent extends through an upperside wall portion of said crankcase.

8. A hermetic reciprocating compressor for a hermetic refrigerationsystem comprising a hermetic shell for receiving suction gas from saidsystem and containing a body of lubricant in the bottom thereof and areciprocating compressor mounted within said shell and having asubstantially closed crankcase at least partially submerged in said bodyof lubricant;

said compressor comprising a cylinder opening into said crankcase and areciprocating piston in said cylinder providing a compression chamber,said piston fitting loosely in said cylinder and thereby permitting somecompressed gas in said chamber to bypass said piston and flow into saidcrankcase, reciprocating movement of said piston in said cylinder alsoeifecting alternating increase and decrease in crankcase pressure;

means for conducting suction gas from said shell to said chamber;

a restricted drain passage interconnecting the lower portion of saidcrankcase with said body of lubricant below the surface thereof and arestricted vent passage interconnecting an upper portion of saidcrankcase with the interior of said shell above the surface of said bodyof lubricant for together conducting said bypass gas to said shell;

said vent having a flow capacity insufficient to vent all of the bypassgas to said shell;

the relative flow capacities of said vent and said drain passagessubstantially preventing flow of lubricant through said drain passage tosaid crankcase during a decrease in said crankcase pressure byreciprocating movement of said piston.

9. The compressor of claim 8 in which said vent passage has a flowcapacity sufilcient to prevent substantial flow of lubricant throughsaid drain passage into said crankcase upon a sudden increase'in shellpressure during operation of said compressor.

10. The compressor of claim 9 in which said vent passage extends throughan upperside wall portion of said crankcase above said cylinder and hasa flow restriction approximately half of that of said drain passage.

References Cited

